My previous U.S. Pat. No. 3,085,197 disclosed an apparatus and method for geophysical prospecting; however, it required the setting up of a number of station sites and establishing a traverse preferably from west to east along a number of such station sites in a given traverse line. At each station a theodolite was set up, leveled, and zeroed in on the traverse line as a zero reference. An electrical circuit was energized and the theodolite head was moved to establish a null both along the dip needle angle and along a horizontal plane. A sensing coil was rotated at a slow speed and readings for the horizontal declination and vertical inclination were obtained together with the time of operation. The theodolite was then removed to the second station along the traverse line and the above procedure repeated. The theodolite was moved from the third to the nth station along the traverse line and again this same procedure was repeated. A second and third traverse line of station sites might be established in order to cover a possible field whereat it was hoped that a hydrocarbon deposit might lie below the surface of the earth. Averages of the inclinations were obtained and averages of the declinations obtained. Deviations from these averages were then obtained and vectors were plotted with the vertical deviation along the Y axis and the horizontal deviation along the X axis. A strike line was drawn along those station sites showing the greatest potential change and which had vectors all pointing generally toward a central location. Readings at station sites along the strike line were taken at two different times during the same day in order to obtain two sets of readings during two different diurnal conditions of the telluric current.
Also disclosed in that patent was a system wherein two separate theodolites were set up at a given station site. The settings for the two vertical inclination readings were set off the null condition on opposite sides and were connected in bucking relationship. Sample readings were taken periodically. Further, it was disclosed in this patent that a plurality of station sites might have the theodolites set up so that a plurality of such stations could have simultaneous recording of the vertical inclination only.
A difficulty with that patented system was the considerable length of time required because only one differential reading per station/day sequence was obtained and also it was necessary to carry an azimuth or to work along a traverse line, which limited flexibility.
My prior U.S. Pat. No. 3,324,385 required the use of an external and movable armature in order to locate the presence of an anomaly below the surface of the earth.
The patent to Pirson et al U.S. Pat. No. 3,943,436 follows the generally accepted procedure that the direction and strength of the earth's currents are indicative of the deeper substrata resistance. Pirson et al discloses the measuring of the total earth magnetic field intensity at a plurality of stations, and requires a closed traverse around a given area on the surface of the earth.
The Reviews of Geophysics and Space Physics, Vol. 18, No. 1, pp. 203-209, Feb. 1980, has an article entitled "Geomagnetic Depth Sounding by Induction Arrow Representation: A Review." This article reviews a number of the previous efforts to measure the upper atmosphere geophysics by the use of arrays of earth surface-based magnetometers. The review starts with Gauss in 1838 and proceeds to the present. However, none of these articles relate to any form of geophysical exploration, but show the present acceptable methods and tools utilized when delineating telluric current induction, measured only by magnetometers.
The article, The Application of Magneto Telluric Soundings to Petroleum and Geothermal Exploration in Non-Seismic Areas: Case Histories, presented at and published in the proceedings of the Twenty-Third International Geophysical Symposium, Varna, Bulgaria October 1978, gives a case history 2 at pp. 6-8 of the article, and this article indicates that it is generally accepted that the direction and strength of the earth's currents are indicative of the deeper substrata resistance. However, most telluric current methods utilize ground electrodes along a closed traverse as a means of obtaining directionally anomalous areas.